One of the basic skills a candidate requires is good mental math abilities. Your interviewer will test this skill either directly by asking a math question such as "What is the square root of 759?" or in an indirect manner while solving a business case problem.
Interviewers can differentiate between hard and soft mistakes. Calculation errors are considered hard mistakes because there is no partial answer in math. Therefore, you must improve or refresh your mental math skills before interviews. Remember, it is alright to use a scratch paper for most calculations but it is expected that you are able to mentally perform easy calculations. Try the following tool to improve your mental math skills.
Possessing excellent mental math skills is crucial during case interviews because you will need these skills as a consultant. In general, there are two main reasons why consultants should possess excellent math skills:
Quant skills are only one of the skills assessed in case interviews. As a candidate, you are evaluated on your overall combination of skills and your perceived potential. Generally, a candidate’s quantitative assessment consists of 20-25% of the overall interview evaluation weight. The goal of Management Consulting firms is to find the set of candidates with what they perceive are the best set of skills overall.
Most firms have a “minimum bar” for quant skills for their candidates, which means you are unlikely to receive an offer if you do not exceed this bar—unless you are otherwise a highly exceptional candidate (such as being a Rhodes Scholar). At some firms, dramatically exceeding this “bar” does not strengthen your candidacy—that is, the quantitative evaluation is binary: you either pass the quant assessment or you don’t. At other firms, such as McKinsey, if you can demonstrate very strong quantitative skills it will differentiate you from other candidates. The quant problems from these firms can be more complex, which allows candidates with strong quant skills to fully demonstrate their ability. The corollary is that, for some very complex quant problems, you don’t need to correctly solve all the components to receive an offer.
The relative importance of quantitative skills in the case interview varies by firm, office and the individual interviewer. As an example, if you are interviewing with a Consulting firm partner who has a PhD in a very quantitative field like Applied Mathematics or Physics, they may ask more challenging quant questions and place a greater emphasis on quant skills in their interviews. McKinsey, in particular, places more emphasis on quant skills than other firms, and, as mentioned previously, demonstrating exceptional quant skills can greatly strengthen your candidacy at McKinsey.
Given the large weighting of 20-25% that quant skills have in consulting interviews - and the quant “minimum bar” at most consulting firms - you can certainly sink your chances of receiving an offer if you perform poorly in the quant portion of the interview.
Quant skills are needed to be an effective Management Consultant; hence, they are assessed in Management Consulting interviews. Quant skills are one of the skills consulting firms assess in their interviews. At most firms, if you do not exceed a “minimum bar” for quant skills, it is unlikely you will receive an offer. You cannot use calculators or spreadsheets in Management Consulting interviews, and the assessment of your quant skills usually contributes about 20-25% of the overall interview evaluation weight.